1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
The exhaust gas of a diesel engine contains particulates comprising carbon as the chief component. It is desired that they are not emitted into the atmosphere. For the purpose, it has been suggested that a particulate filter to trap particulates is arranged in the exhaust system.
When the particulate filter traps particulates, resistance to the exhaust gas in the particulate filter increases gradually and thus it is required that the trapped particulates are removed before the resistance becomes very large. For this purpose, Japanese Unexamined Patent Publication No.2001-271633 suggests that the particulate filter carries an active-oxygen releasing agent which holds NOx to allow, the NOx to combine with oxygen when excessive oxygen is present in the surroundings, and releases and decomposes the combined NOx and oxygen into NOx and active-oxygen when the oxygen concentration in the surroundings drops. The active-oxygen released from the active-oxygen releasing agent can oxidize the trapped particulates favorably on the particulate filter. Thus, the trapped particulates can be automatically removed, Besides, it is also desired that NOx included in the exhaust gas of the diesel engine is not emitted into the atmosphere. Accordingly, if this particulate filter is used, NOx in the exhaust gas is absorbed in the active-oxygen releasing agent and thus NOx can be prevented from being emitted into the atmosphere.
By the way, the active-oxygen releasing agent also holds SOx in the exhaust gas as it does NOx. The held SOx is not released even if oxygen concentration in the surrounding merely drops. Accordingly, the amount of SOx held in the active-oxygen releasing agent on the particulate filter continues to increase. When an amount of held SOx increases (this is referred to as sulfur poisoning, below), an amount of NOx that can be held in the particulate filter decreases. The above-mentioned particulate filter is intended to purify NOx with the oxidization of the trapped particulates. Therefore, if the particulate filter cannot hold NOx due to the sulfur poisoning, the purification of NOx becomes insufficient.
To regenerate the sulfur poisoning from the particulate filter, the oxygen concentration in the surrounding is made low and the temperature of the particulate filter is made high. The capacity of the particulate filter is relatively large and thus a large amount of energy is required to heat the particulate filter such that the temperature of the whole particulate filter is made high. Besides, when the temperature of the particulate filter becomes high, the carried active-oxygen releasing agent and noble metal catalyst is deteriorated by the heat.
Accordingly, it is preferable that the sulfur poisoning does not occur in the particulate filter. Japanese Unexamined Patent Publication No.2001-27114 suggests that SOx trap means is arranged upstream of the catalytic apparatus to trap SOx before it flows in the catalytic apparatus. Even if an SOx trap means is used, it cannot continue to trap SOx limitlessly and, thus, it is required that SOx must be released therefrom when an amount of SOx trapped therein reaches a predetermined one. At this time, it is suggested, the exhaust can gas bypass the catalytic apparatus such that the released SOx does not flow in the catalytic apparatus.
Of course, when SOx is released from the SOx trap means, the whole of the SOx trap means must be heated. The SOx trap means has a capacity smaller than the particulate filter and thus the energy consumption when the whole SOx trap means is heated can be made smaller than that when the whole particulate filter is heated. However, a relative large energy consumption is still required when the whole SOx trap means is heated.